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Superheat! Pages: 12

Edypaul on Fri August 19, 2011 11:55 AM User is offline

Year: 97
Make: chevy
Model: full size van
Engine Size: 304
Refrigerant Type: 134a
Country of Origin: United States

First off, This is a great forum! I,m glad I found it. I think Ive read every post on this fourm and have learned a bunch from it. Thx . I,m an A&P by Trade and an amateur A/c tech by choice. Here is my question. Can you check auto A/c (ot sys) sys perfomance by using superheat?? Taking in to account temp and humidity?

Thx Ed

Edited: Fri August 19, 2011 at 6:18 PM by Edypaul

bohica2xo on Fri August 19, 2011 2:46 PM User is offline


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edypaul on Fri August 19, 2011 4:08 PM User is offline

The reason I ask is that their are books out there that teach thet you can. Ref. auto a/c In a home a/c sys the compressor runs at a constant speed and is in a different enviromnt but the only way to get a correct charge is with superheat or subcool. I have always used a 30 lb can and a scale to service my A/c sys with the recommended amount of freon. What does a guy do if its a bastard sys or does not have the spec to charge it too. wy wont superheat work?

Dougflas on Fri August 19, 2011 4:33 PM User is offline

Reread your own post. You already answered your own question. :-) Automotive AC systems do not run at constant speeds. Best way is to weigh the charge. Actually. residential systems should be charged by weight also for most accurate results. Take a package unit..the pipe lenghts are a constant size. The manufacturer calculates the charge and uses the weight syste. If one could/would calculate a resi split system's pipe lengths, they could be charged by weight also. No one does this. So we are left with charging by superheat and subcool methods because airflow affects superheat. Also, the accuracy of the measuring tools affects the end result. Also. resi charging methods specify a +/- 3*.

NickD on Fri August 19, 2011 5:57 PM User is offline

Originally posted by: Edypaul
The reason I ask is that their are books out there that teach thet you can. Ref. auto a/c In a home a/c sys the compressor runs at a constant speed and is in a different enviromnt but the only way to get a correct charge is with superheat or subcool. I have always used a 30 lb can and a scale to service my A/c sys with the recommended amount of freon. What does a guy do if its a bastard sys or does not have the spec to charge it too. wy wont superheat work?

Describe how you would charge a home HVAC system with superheat, then I will be happy to post how an automotive system differs.

Edypaul on Fri August 19, 2011 5:57 PM User is offline

All right . That just about sums it up. Thanks for the info. A/c systems are pretty amazing machines. The more I learn about them the more I,m fascinated by the way they work. Can anyone recommend any good a/c tech books? Call me crazy but I love to read about this stuff.

Thx Ed

Edypaul on Fri August 19, 2011 6:05 PM User is offline


I don't have the reference in front of me now but from what I recall , first you measure wet bulb temp and dry bulb temp inside the house measure outside temp. Then you measure evap discharge temp and pressure. Find the numbers off a temp press chart then do the math.There is a formula to figure out what the superheat should be. Should be somewhere about 10 or so degrease.

Edited: Fri August 19, 2011 at 6:12 PM by Edypaul

70monte on Fri August 19, 2011 7:14 PM User is offline

I have a couple of Auto AC books. The first one is by Mastercool and its called "Automotive Air Conditioning Basic Service Manual" I think this site sells it.

The other one is from Haynes "Techbook" series of books called "Automotive Heating & Air Conditioning" It also has pretty good info and it includes refrigerant capacities and pressures up to about 1999.

They both describe how these systems work and how to troubleshoot and repair them.

There might be some others out there but I don't know what they are.


Edypaul on Fri August 19, 2011 8:00 PM User is offline


Thanks for the info. I have seen the Hanes manual. It was pretty basic. I will checkinto the Mastercool. Im looking for something with a little more theory. The problem is that their is a lot of conflicting information out there. I would like to get ahold of some textbooks from a school.

Thx Ed

mk378 on Fri August 19, 2011 9:17 PM User is offline

CCOT systems such as almost all American cars and trucks operate at zero to negative superheat. The evaporator is supposed to flood, but the accumulator traps liquid refrigerant from entering the compressor. This is entirely different from how home systems with a capillary tube or TXV operate.

Proper procedure is to check the sticker or service manual and charge by weight to specified amount.

Edited: Sat August 20, 2011 at 10:43 AM by mk378

mk378 on Fri August 19, 2011 9:18 PM User is offline

Edited: Fri August 19, 2011 at 9:19 PM by mk378

70monte on Fri August 19, 2011 9:26 PM User is offline

I did a search for mobile AC books and there are quite a few but the ones I did find are pretty pricy but they probably have the info you are looking for.

I would like to take an Auto AC class at some point so I can learn about how to diagnose the electrical side of AC problems. That is the part I have very limited knowledge on.


bohica2xo on Fri August 19, 2011 9:28 PM User is offline

Variable speed evaporator blower.

Variable compressor output based on shaft speed, sometimes also based on evaporator suction pressure.

Variable condensor airflow with no relationship to either compressor speed, evaporator airflow, or ambient temperature.

Now build yourself a 3 dimensional map of the entire operation envelope, much like the ones used for EFI injector pulsewidth calculations. On a TXV system, the superheat is always someplace inside that solid model. With a CCOT, it is not even inside the envelope...

Hence the 4 letter answer. Well, that and I was on a phone at the airport...


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Dougflas on Fri August 19, 2011 9:41 PM User is offline

Down and dirty resi superheat charging with no needed chart

Superheat =Indoor wet bulb temp X 3 minus outdoor drybulb-80. Take this total and divide by 2

Superheat = (IDwetbulb X 3) - outdoor Drybulb - 80



Edited: Fri August 19, 2011 at 9:43 PM by Dougflas

JJM on Fri August 19, 2011 9:49 PM User is offline

As everyone has chimed in, with an OT system no, but with a TXV system and no evaporator pressure controls (variable displacement compressor, EPR, POA, STV), you could check system performance by superheat. And as MK378 correctly pointed out, OT systems work on zero to negative superheat, which is why superheat testing won't work; if you had any degree of superheat on an OT system, that would indicate an undercharge.

Also as DougFlas pointed out, systems need to be charged by weight... easiest troubleshooting tool, yet so many folks try to charge by pressure. Of course, often that's all you can do with a retrofit, though on CCOT systems you zero and slightly negative superheat confirms a proper charge. This is our MVAC world.

Of course, with HVAC, superheat is a an excellent way to determine charge and diagnose... but I bet if 9 out of 10 residential HVAC techs here in NYC never use it.


When considering your next auto A/C purchase, please consider the site that supports you:

NickD on Sat August 20, 2011 7:57 AM User is offline

Just happen to have a 1st class FCC license and also a certified FAA electronic tech. Have to admit that the A&P's I work with are lost when it comes to the more complicated end of aircraft electrical systems, definitely lost in avionics. And also find that to be true with AC pros when it comes to the electronic/electrical end.

Work as a non-fed, have to have all my own test equipment, and a lot of it, all traceable to the NITS, find it amazing that employed FAA personal are not even required to have a general FCC license, also happened to be an electronic engineer that required a ton of study and experience. Also have to be nice and get along, even though I am working for idiots.

Home HVAC systems are very simple electrically, thermostat closes, switches on a contactor, and it runs, a separate contactor switches on the blower motor, system doesn't even know if the blower is running or not, so sensors for that. Have an older system with a low pressure cutout, see they are not even using those in the newer systems now. AC can run for days with zero charge. Getting zero superheat in these systems is easy, just pull the blower motor out. Of course the evaporator will freeze up in the process. Its temperature is not even monitored.

Charging a home AC system is very similar to charging a MVAC system, if you want to run your system a full rated rated capacity for also efficiency, have to follow the manufacturers recommended charging by using PT charts and charging by pressures. Yes you can lower the charge for less superheat, but the consumer, me, in this case is paying a much higher electrical bill. To me charging is not a variable, its the capacity of the system, proper sizing the of ductwork, where the blower speed becomes the variable, primarily to prevent the system from freezing up. In energy tests, like to run the blower at a higher speed, increases superheat, but what the hell, my power measuring equipment and BTU measurements tell me I am using less overall electricity. Motor output power is not linear with load, so if a charge is low, using a lot more energy for essentially doing nothing.

For years, used the same gauges with the same fittings for both R-12 and R-22, just took that for granted, but not anymore. At one time you were suppose to have brains not to put R-22 in an R-12 system, but still don't need brains for that, can buy adapters.

Require EPA 608 or 609 certification to work on these systems, tests were also written by idiots, don't use logic, but have to look up the answer they want so you can get your certification. And you sure in the hell don't have to know a damn thing about AC to pass these tests.

Since we both work for the FAA, have to do everything by the book, with my engineering background certainly cannot do any modifications to improve the system like guidescope or ILS, have to follow the book or will be in deep trouble. Met an FAA agent that wrote part of the book, didn't have an engineering degree nor even a general radio license, just took bits and pieces from service manuals and tacked it all together with no consideration as to the types of modulation systems incorporated. While I knew the adjustment I was making was incorrect, followed his book anyway. Not optimum, but still worked. The guy in charge of non-feds doesn't know the difference between a capacitor and a resistor, but did help Bush get elected.

But if you like following the book, get a shop manual on your vehicle and follow that.

Readying a lot of AC books, authors didn't take a college course on physics with a lot of misinformation on Boyle's law

Gerald K4NHN on Sat August 20, 2011 4:43 PM User is offlineView users profile

NickD, interesting read on your back ground.

Gerald K4NHN & "ole FCC 1ST Class"
Cayce, SC

Edypaul on Mon August 22, 2011 7:26 PM User is offline

Well! That sheds a light on why you cant charge an Auto a/c with superheat. Makes sense. Yea when I got into the airplane business you had to have an FCC license to work Avionics. That went away back in the 80s. Very few of the guys I work with know the difference between a cap and a res. Thats ok thou because their not working on the bench. We closed are Avionics overhaul shop years ago and most of the guys that worked there have retied. (except me). The FAA. Thats another story. Most of those guys don't know there a** from a hole in the ground. Its just another large gov bureaucracy.

My problem is I have a 97 chevy full size conversion van. (duell sys). 134a The charge sticker says charge to 72 oz. There is a sticker from the conversion company also that says to charge an extra 7 oz to the says. I think I saw a chart for that year vehicle that says to charge to 78 oz. Any thoughts on this??

Edited: Mon August 22, 2011 at 7:28 PM by Edypaul

newton5 on Tue August 23, 2011 2:03 AM User is offlineView users profile

Your van started life as a cargo van and was sent to an upfitter, who added their rear AC unit along with the interior upfit and body/paint/detail modifications. You would use the 72oz plus 7oz recommendation, or 79oz.
The 78oz recommendation is for vans with factory installed rear AC.
Actually, the 1 ounce difference in a system that large is a non issue.

NickD on Tue August 23, 2011 5:54 AM User is offline

Here is a P-T chart from the old air conditioner board that went away leading Tim to start this board.

Actually went out in -25*F weather to replace a defective Scharder valve with hardly losing any refrigerant, that was a long time ago. But you can read the chart in reverse, at 28.5 psi low side pressure, the refrigerant will vaporize to a 30*F temperature, with R-12. For R-134a its 26.1 psi for 30*F. These are the numbers I try to hit at 85*F, doors open, AC on, blower at max, engine running at 1,500 to 2,000 rpm. But have to wait for an 85*F day to do this for optimum charging, a good reason why to charge by weight.

Your gauges should have a P-T scale, find the chart a bit easier to read.

Also at 85*F that is when you want pure liquid to flow in the high side line hitting the orifice, less will give you foam, that doesn't cool too well, more will kick your high side up to the limit of kicking out the high pressure cutoff switch. That 30*F won't be your vent temperature, have thermal resistance between the refrigerant and evaporator with additional resistance between the evaporator and air flow through it. Would be lucky to get vent temperatures in the 40*F range, but this only holds true if the air paths are clean. An evaporator acts like a furnace air filter and can get quite disgusting with debris buildup. So the first step is cleaning up that mess first. They don't make this easy in automotive. You can call this difference in temperature superheat if you want. Some later vehicles are equipped with an inlet air filter, but that also gets dirty. A good MVAC will not freeze up with restricted air flow, but will cycle more frequently or simply be turned off. Most HVAC systems do not have these sensors and will freeze.

Ha, got a call from a friend telling me their home system froze up and won't blow air. Asked him when was the last time he changed his filter. What filter. Change your filter, switch your thermostat to off, put your blower on manual to dry it out and see what happens. It worked for him.

But if that happens here, oh, I must be low on refrigerant, have to run to Wal-Mart and buy a kit.

Vehicles do have volume tolerances, so charging by weight is more of a production average, but can either be over or undercharged for optimum performance. One variable is the accumulator or receiver that can store excess refrigerant. Recommended charging is to half full that accumulator or receiver, primarily for warranty reasons if you do have a slow leak. Will use up that excess first.

You can see a wide variation in pressures versus temperatures where in a vehicle the compressor will operate in a 35 to 125*F ambient. CCOT are the worst and should be outlawed, the hotter it gets, the less cooling you get. But also the cheapest system to manufacturer. A once good POA system would hold that 30 psi over the entire temperature range. Where a CCOT system can see low side pressure upwards toward 90*F in a 130*F environment. You don't get much cooling at these high pressures.

You probably noticed a lot of inspection plates on an aircraft for an annual inspection. Won't find this in a land vehicle, again due to economics. Worse part of MVAC work is cleaning the systems, start off with an evaporator and build a whole car around it. But is the first and most important step in MVAC work.

Edypaul on Tue August 23, 2011 9:58 AM User is offline

O.K. I,ll go for the 72 oz charge and see what the numbers look like. It will be a while before it gets down to 85 egress here in florida. It averages 90 to 100 with high humidity in the summer months. Maybe this hurricane that is coming will cool things off a little bit! I,am taking a shot at resealing my HT6 compressor. It was leaking around the case O rings. its back together except for the double lip seal. I,am waiting for the seal protector to install the new seal. Thanks for all the great info. I,ll let you know how it turns out.

Thx Ed

NickD on Tue August 23, 2011 1:42 PM User is offline

Do you mean Irene? She really sounds mean, forecasting a category 4. Think I will be more concerned about tying things down then charging an AC system. Good luck with that.

Edypaul on Tue August 23, 2011 3:20 PM User is offline

I hate it when the forecasters come out with the track model forecasts. It seams like everytime they forecast a storm ,its heading for florida. So me , my wife thinks im a nut, runs out and stocks up on gas and plywood. Then the forecast changes and were ok. Ive got 40 gallons of gas for the generator! The good part is I can always put it in the car. In 2004 Charley come right over our house . We were without power for a week.! It wasn't pretty. I swore I would never get caught again without a gen. So now Irene is going to miss us. Go figure. Better safe then sorry. I guess I,ill be working on the Van this weekend.

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